- When amount of context exceeds the threshold, the `NullContext` will be set to current thread local. Thus, when checking context in `CtSph#entry`, once `NullContext` detected, the entry will not do rule checking on the slot chain.
- Cache the default context during initialization. Then when amount of context exceeds the threshold, entries under default context can do rule checking under default context.
- Enhance the error message

Signed-off-by: Eric Zhao <sczyh16@gmail.com>